LEDs have the inherent potential to provide the brightness, output, and operational lifetime that would compete with conventional light sources. Unfortunately, LEDs produce light in semiconductor materials, which have a high refractive index, thus making it difficult to efficiently extract light from the LED without substantially reducing brightness, or increasing the apparent emitting area of the LED. Because of a large refractive index mismatch between the semiconductor and air, an angle of an escape cone for the semiconductor-air interface is relatively small. Much of the light generated in the semiconductor is totally internally reflected and cannot escape the semiconductor thus reducing brightness.
Previous approaches of extracting light from LED dies have used epoxy or silicone encapsulants, in various shapes, e.g. a domed structure over the LED die or formed within a reflector cup shaped around the LED die. Encapsulants typically have a higher index of refraction than air, which reduces the total internal reflection at the semiconductor-encapsulant interface thus enhancing extraction efficiency. Even with encapsulants, however, there still exists a refractive index mismatch between a semiconductor die (typical index of refraction, n of 2.5 or higher) and an epoxy encapsulant (typical n of 1.5).
FIG. 1 shows another approach for providing an LED with improved light extraction efficiency (U.S. Patent Application Publication No. U.S. 2002/0030194A1) (Camras et al.). This approach uses a transparent optical element 2 having a refractive index greater than about 1.8, bonded to an LED die 4. A disadvantage of this approach is that when the optical element 2 is bonded to the LED die 4, the bonded system incurs stress forces from each element expanding as it heats up during operation.
LEDs need to be operated at a relatively low junction temperature, typically no more than 125 to 150° C. This limits the maximum current flow and, correspondingly, the output of the LED. Poor heat management can also adversely impact LED lifetime by causing the LED die to run hotter than desired at a given current. Enhancing heat extraction from the LED die can increase the driving current thus providing higher light intensity and longer lifetime. Known methods of extracting or dissipating heat from the LED die include extracting heat through the base of the LED die (typically the side opposite the primary emitting surface). Other methods include adding a heat dissipating fluidic coolant to the LED package, for example as described in U.S. Pat. No. 6,480,389 (Shie et al.).